Satgen426  Spread Spectrum QRM ?  by GM4IHJ  (BID SGEN426)   23 May 97

There are two basic ways to modulate a radio signal with information.
You can use narrow band modulation as with systems like SSB, PSK and
even FM. Restricting the signal to a relatively narrow band of
frequencies but using a lot of power in the narrow sidebands. Or you
can spread the signal out thinly like jam in a sandwich, covering 500
KHz or more of bandwidth but spreading out the power distribution so
that the power in each few KHz is ( or should be ) very low.

Spread Spectrum modulation of signals has been around for a very long
time, but the arrival of digital systems has suddenly given it much
greater prominence. It is being proposed in some quarters for amateur
radio use, and it has already appeared in several military systems and
is being canvassed for many forthcoming commercial systems. Not least
amongst its "qualities" , is the suggestion that it can easily share
with (overlay), existing systems of modulation without ill effect to
them or itself. Is this true ? Recent experience suggests it is not.

One of the earliest examples of the worldwide use of spread spectrum
occured in the late 1970s and early 1980s, when the Soviets were
conducting Killer satellite tests. Far from being covert, the signals
were very obvious. Stretching over 500 KHz of bandwith in the radio
amateur 2m band they produced a forest of carriers roughly 1.5 KHz
apart dopplering across the band at considerable signal strength at
any receiver in the satellites footprint. They were a nuisance but
they never became frequent enough to constitute a major problem.

Unfortunately modern spread spectrum systems introduced unannounced,
are already apparently going a long way towards ruining the 70 cm
amateur band and its adjacent spectrum over the British Isles . The
signals appear to come from the masts of the high power UHF stations
which radiate the 5 channels of the UK terrestrial television service.
But the signals are clearly not a product or cross modulation artifact
of the TV signals . They appear to be totally separate military or
government traffic.

As seen on a spectrum analyser 14 kms from one of the TV masts, the
noise level background appears to rise +10dB over a band of
frequencies which stretches all across 430 to 440 MHz and some way
beyond at either end. The signals are apparently being radiated from
several widely separated TV masts in various parts of the UK, and they
are destroying radio amateur low signal strength experimental work
such as Moonbounce, Mars vehicle tracking, Sporadic E, Meteor Scatter
, FAI and Auroral studies , over a wide arc of bearing. Transmission
power appears to be several Kilowatts and the signals seems to be data
with pseudo random noise modulation. This British experience is
salutory . Clearly "Sharing" bands with authorities who operate in
this way is an impossibility. The Spread spectrum takes over the whole
band, blatantly ignoring other band users. A situation which becomes
highly significant in view of the recent suggestions that radio
amateurs accept sharing with spread spectrum users on several of their
bands, as presently being discussed by at least one national
authority.